Numerous environmental and iatrogenic pro-oxidative stressors ranging from chemotherapy to ultraviolet radiation have been demonstrated to induce immunosuppression in preclinical murine models. These stressors have also been shown to be potent inducers of the lipid mediator platelet-activating factor (PAF), which exerts immunosuppressive effects in murine models. Ours and other groups have characterized the exact pathways by which PAF-dependent immunosuppression occurs and have identified a major role of regulatory T cells (Treg). PAF released from injury acts upon the mast cell PAF receptor resulting in the formation of immunosuppressive Tregs. This process is dependent upon the cytokine IL-10 and cyclooxygenase-2 (COX-2) generated prostaglandins. Though high levels of PAF has been shown to be generated in humans in response to many of the immunosuppressive stressors that induce PAF in animal models, studies are needed to define if this PAF?COX-2?Treg pathway is functional in humans. This revised R21 application is designed to take advantage of our group?s recent findings that topical photodynamic therapy (PDT) is a very potent inducer of PAF in murine skin, and induces systemic immunosuppression only in mice expressing PAF receptors. PDT is a very common procedure in dermatology which has been shown to exert immunosuppressive effects in humans. This provides the rationale to test if the PAF?COX-2?Treg pathway is involved in PDT-induced effects and could be pharmacologically modulated with COX-2 inhibitors to block the immunosuppression and potentially improve PDT efficacy in treating precancerous actinic keratoses lesions. We will test if standard PDT performed in dermatology clinics generates: 1) PAF and related oxidized glycerophosphocholines in skin; 2) systemic isoprostanes in response to reactive oxygen species; and 3) systemic COX-produced prostaglandins. We will also characterize the immunosuppressive effects of PDT in humans by both skin testing, and assaying Tregs and T cell responses against established and neo-antigens. The ability of short-term post-PDT treatment with the COX-2 inhibitor celecoxib to modulate PDT-mediated immunosuppression and clinical effectiveness will also be defined. These translational studies will thus define if a previously unappreciated immunomodulatory pathway is engaged following PDT which can be exploited to enhance treatment outcomes. If successful, these studies will change the standard of care for how PDT is administered.